In general, the aerodynamic efficiency of any lifting surface, regardless of the type of vehicle, is dependent on the lift-to-drag ratio of that surface. Various methods for controlling aerodynamic surfaces on rotor blades, wings, engine inlets, fan blades, and nozzles are known. Movable control surfaces placed on these aerodynamic surfaces have included flaps, slats, spoilers, ailerons, elevators, and rudders. Although these control surfaces can mechanically alter the geometry of the original aerodynamic device, they are limited in their ability to respond quickly and efficiently. Furthermore, such mechanical control surfaces may have a number of disadvantages, including adding complexity to the aerodynamic device, reducing structural integrity, complicating manufacturing, and compromising radar detectability.
Therefore, aerodynamic surfaces such as aircraft wings, helicopter blades or windmill blades are designed to operate efficiently at conditions that maximize lift and minimize the attendant drag penalty. Under certain operating conditions, for example at high angles of attack, boundary layer separation occurs resulting in a loss of lift and a simultaneous increase in drag, thereby compromising the aerodynamic efficiency of the surface. In recent years the use of active flow control as a means to improve aerodynamic efficiency of a surface over a wide range of operating conditions (e.g., varying Mach numbers and Reynolds numbers) has met great success. Numerous wind tunnel investigations have shown that significant aerodynamic benefits are achievable through the use of low momentum oscillatory or pulsed jets. These benefits include improved stall and post-stall lift characteristics which offer simultaneous reductions in drag. For a typical rotor blade or wing, these benefits translate into an increase in useful payload, reductions in power requirements resulting in fuel savings, or an increase in aircraft range for the same power.
Prior successful attempts to achieve some of these advantages have incorporated devices known as synthetic jet actuators into various aerodynamic surfaces, for example, helicopter blades. A synthetic jet includes a movable diaphragm or piston positioned within a pump chamber. Movement of the diaphragm or piston pulses air in and out of the chamber through an orifice. In the context of a wing or blade, the moving member is positioned within a hollow portion of the air pump actuator structure and pulses air in and out of one or more orifices in the outer aerodynamic skin. The outer skin thus may be made relatively porous and the wing or blade may have a plurality of such synthetic jets incorporated therein for active flow control. See, for example, U.S. Pat. Nos. 5,813,625; 5,938,404 and 6,471,477 each of which are incorporated herein by reference.
The prior art has also utilized electromagnetically-driven air pumps to generate oscillatory or pulsed jets but these have demonstrated limited reliability due to thermal limitations and shortened life cycles. These devices overheat due to the electrical energy needed for cyclic operation of electromagnetically-driven prior art air pumps.
A further limitation of such devices involves the displacement and frequency requirements for delivering pulsed blowing or suction configurations and is also limited by weight penalties and speed constraints. A typical electromagnet pulsed jet cannot deliver adequate flow at a high frequency on the order of 300 Hz. These prior devices generate and convey local heat energy to the surface port but the thermal energy cannot be dissipated quickly enough for satisfactory performance.
The subject ultra-low friction air pump design improves upon these prior designs and can convey thermal energy away from the surface in a more efficient manner, particularly when used in conjunction with pneumatic or hydraulic systems instead of the aforementioned electromechanical systems. Excess heat energy can be removed by conventional means such as by a heat exchanger or by conduction through supply lines, but in general, the frictional heat generated from the subject ultra-low friction air pump is minimal due to the improved frictional properties of the oscillating components.
Additionally, the ultra-low friction air pump also provides improvements in useful life cycle operations for such devices.
Thus, a primary objective of this invention is to present a ultra-low friction design for an air pump device for generating oscillatory or pulsed blowing or suction jets.